In recent years, there have been the increasing demands, in the market, for a silver halide photographic light sensitive material having a high photosensitive speed and an excellent graininess. In response thereto, the silver halide crystals applied to light sensitive materials have been controlled more precisely and complicatedly.
From the viewpoints of photosensitive speeds, graininess, developability and so forth, the so-called core/shell type grains have widely been used previously, in which the halide compositions have been changed between the inside of silver halide grains and the outside thereof, such as those disclosed in Japanese Patent Publication Open to Public Inspection (hereinafter abbreviated to JP OPI Publication) Nos. 57-15432/1982, 60-143331/1985, 60-138538/1985, 58-9137/1983, 58-9573/1983, 59-48755/1984 and so forth.
Because these emulsions have such a theoretical problem as mentioned hereunder in the processes of forming the grains thereof, it has therefore been found that there are some limitations, from the viewpoints of improving the emulsion characteristics (including, particularly, a photosensitive speed, graininess and fogginess). To be more concrete, in the conventional grain-forming processes, silver ions and halide ions, necessary for forming grains, are each supplied in the forms of an aqueous silver salt solution and an aqueous halide solution into a mixer or a reaction chamber, respectively. Therefore, the excessively concentration distributions of the aqueous solutions may be produced around an adding nozzle and a mixing propeller. The presence of a region having such an ununiform concentration as mentioned above may lead to a rapid growth of silver halide grains passing through the region. Resultingly, reduced silver may be produced and the microscopic unevenness of the halide compositions may also be produced in the grain-forming processes.
As for the already known methods for fundamentally overcoming the problems retaining unsolved in the conventional processes for forming silver halide grains, there include the methods such as disclosed in, for example, W/O 89/06830 or JP OPI Publication No. 2-166442/1990, wherein grains are formed by adding finely sized silver halide grains (hereinafter sometimes referred to as silver halide fine grains or simply to as fine grains) and then by ripening them.
The above-mentioned method is characterized in that silver halide fine grains are used as the supply sources of silver ions and halogen ions. This method is also regarded as the method for forming grains, in which such a problem as mentioned above cannot be raised theoretically, because the ions are released continuously when the fine grains are dissolved after adding and dispersing them into a reaction chamber and because the ion concentration inside the reaction chamber is uniformed by the tremendous numbers of the fine grains.
In the grain-forming methods in which the above-mentioned fine grains are added, the groups of the methods each consisting of a method for forming the fine grains themselves and another method for adding the fine grains thereafter may be roughly classified at present into the following two known methods.
(a) One method in which an aqueous silver salt solution and an aqueous halide solution are reacted each other in a mixing chamber separated from a reaction chamber for forming grains, so that silver halide fine grains having a desired halide composition can be formed, and the resulting silver halide fine grains are immediately added into the reaction chamber.
(b) The other method in which separate from a grain-growth process, silver halide fine grains having a desired halide composition are prepared in advance and the resulting fine grains are added when the grains are grown.
The expression, `a desired halide composition`, stated in each of the above paragraphs (a) and (b) means the same as the halide composition of an objective phase made present inside the silver halide grains to be formed. The above-defined meaning is regarded to be preferable.
The silver halide emulsions prepared each by following the above-mentioned methods and the other silver halide emulsion prepared in the conventional methods (such as a double-jet method in which an aqueous silver salt solution and an aqueous halide solution are used) are each compared and evaluated on the photographic characteristics thereof. However, it was resultingly proved that almost no advantageous effect could be achieved particularly concerning the reduction of fog production (or a reduced silver production) and the improvements of graininess, although the methods are regarded as the grain-forming methods capable of theoretically solving the aforementioned problems.
As for the methods for forming silver halide grains in the presence of an oxidizer, there are known techniques disclosed in for example, JP OPI Publication Nos. 61-3136/1986, 62-54249/1987, 62-73251/1987, 3-172836/1991, 3-189641/1991 and 3-196138/1991. The objects of these techniques are to oxidize reduced silver (or metallic silver) produced chiefly when forming grains so that a fog production can be inhibited. However, even when applying these techniques, it was proved that any satisfactory effects could not be achieved on the fog reduction and the improvements of graininess corresponding to the fog reduction.